A CRT for generating color pictures generally contains an electron gun emitting three coplanar beams of electrons (R, G and B electron beams), to excite on a screen a luminescent or phosphorous material of a given primary color red, green, and blue, respectively. The deflection yoke is mounted the neck of the tube for producing deflection fields created by the horizontal and vertical deflection coils or windings. A ring or core of ferromagnetic material surrounds in a conventional way the deflection coils.
The three beams generated are required to converge on the screen for avoiding a beam landing error called convergence error that would otherwise produce an error in the rendering of the colors. In order to provide convergence, it is known to use astigmatic deflection fields called self-converging. In a self-converging deflection coil, the field nonuniformity that is depicted by lines of flux generated by the horizontal deflection coil has generally pincushion shape in a portion of the coil situated in the front part, closer to the screen.
A coma error occurs because the R and B beams, penetrating the deflection zone at a small angle relative to the longitudinal axis of the tube, undergo a supplementary deflection with respect to that of the center G beam. With respect to the horizontal deflection field, coma is generally corrected by producing a barrel shape horizontal deflection field at the beam entrance region or zone of the deflection yoke, behind the aformentioned pincushion field that is used for convergence error correction.
A coma parabola distortion is manifested in a vertical line at the side of the picture by a gradual horizontal direction shift of the green image relative to the mid-point between the red and blue images as the line is followed from the center to the corner of the screen. If the shift is carried out toward the outside or side of the picture, such coma parabola error is conventionally referred to as being positive; if it is carried out toward the inside or center of picture, the coma parabola error is referred to as being negative.
A geometry distortion referred to as pincushion distortion is produced in part because of the non-spherical shape of the screen surface. The distortion of the picture, referred to as North-South at the top and bottom and East-West at the side of the picture, is stronger as the radius of curvature of the screen is greater.
When the screen has a relatively large radius of curvature greater than 1 R, such as 1.5 R or more, for example, it becomes more and more difficult to solve the beam landing errors, such as the geometry distortion, without utilizing magnetic helpers such as shunts or permanent magnets. For example, in the prior art deflection yoke of FIG. 2, permanent magnets are positioned in front of the deflection yoke to reduce North-South geometry distortions.
It is common practice to divide the deflection field into three successive action zones along the longitudinal axis of the tube: the back or rear zone closest to the electron gun, the intermediate zone and the front zone, closest to the screen. Coma error is corrected by controlling the field in the rear zone. Geometry error is corrected by controlling the field in the front zone. Convergence error is corrected in the rear and intermediate zones and is least affected in the front zone.
It may be desirable reduce the North-South geometry distortion by controlling winding distributions of deflection coils without utilizing magnetic helpers such as shunts or permanent magnets. Eliminating the shunts or permanent magnets is desirable because, disadvantageously, these additional components may produce a heating problem in the yoke related to higher horizontal frequency, particularly when the horizontal frequency is 32 kHz or 64 kHz and more. These additional components may also, undesirably, increase variations among the produced yokes in a manner to degrade error such as geometry, coma, coma parabola or convergence error corrections.
In the prior art deflection yoke of FIG. 2, a separator is composed of a main part 161 conforming to the shape of the tube on which the deflection yoke is mounted for a substantial length of the seperator. However, a front end 160 of the separator deviates away in a plane perpendicular to the Z axis from the funnel shape contour of the tube. An inside surface of front end 160 is used for supporting front end turn of the horizontal deflection coil. The circular shape of inside boundary 162 of front end 160 forms a boundary between part 160 that is perpendicular to the Z-axis and part 161 having the flare shape that conforms to the conical shape of the tube funnel. The surface of the wall of the flared front end 160 of the separator is flat and perpendicular to the main Z axis. During the winding process of the coil, retractable pins are inserted perpendicular to the XY plane to form corners in the winding. In the prior art yoke of FIG. 2, pins are placed substantially at boundary circle 162 of front end 160 of the separator, between parts 160 and 161.
It may be desirable to utilize front end 160 for increasing the effective length of the coil. Increasing the effective length of the coil facilitates shifting the deflection center of the horizontal deflection coil with respect to that of the vertical deflection coil.
In accordance with an inventive feature, the corners in the winding produced by the pins are placed remote from the boundary circle. Thereby, advantageously, a substantial portion of the coil is extended in the front end 160. The result is that the effective length of the coil is increased in a manner to reduce North-south geometry distortion.